A data centre is a late 20th Century development that has grown as a response to the increasing demand for computer processing capability and a recognition of the importance of IT in the place of every business and organisation today. Whereas smaller organisations have sufficient processing power with laptops, PCs and occasionally servers, larger organisations require higher capacity centralised processing to serve a wide range of needs and applications. A few years ago this capacity was supplied by large mainframe computers, but more recently the method used has been to provide data centres comprising many networked computer servers known as blades installed in racks enabling controlled and modular expansion of capacity. The racks also typically house telecommunications equipment such as routers to handle data flow between the computer servers and data flow between the data centre and the outside world.
Data centres can mirror the growth and business activities of successful companies. The growth of a data centre within in an expanding company may typically work as follows:
1. Initially the data centre may start as single rack of servers in an air conditioned room—sometimes referred to as a ‘data closet’.
2. As the organisation expands and along with it the number of IT racks employed increases, the closets become ‘Server Rooms’ or ‘IT Rooms’.
3. Eventually the number of racks and size of room expands, often to the point where a dedicated building or part of a building houses the IT. Whilst there is no strict definition of when the size of an IT facility becomes large, or sophisticated, enough to be termed a “data centre”, data centres are typically relatively large IT facilities providing robust and resilient IT facilities. Typically, there will be more than 50 servers (often many more) and at least some redundancy in the power supply powering the servers to ensure continuity of service.
4. As the company grows and/or becomes a multi-national organisation additional data centres will be built and sometimes numbers of these will be consolidated into ‘Super Data Centres’.
Data centre facilities can require a floor space ranging from a few hundred square feet to a million square feet. The most prevalent size for a small data centre is five to ten thousand square feet with fifty to a hundred thousand square feet being the most common floor area requirement for a large data centre.
As data centres grow, improved methods for efficient cooling of the servers have become more sought after. In air-cooled data centres, servers are typically arranged in rows of racks separated by alternating ‘hot’ and ‘cold’ aisles. Cooling air is supplied to the cold aisles, often from an underfloor plenum via perforated floor tiles, and then drawn into the servers in racks adjacent to the cold aisle by internal server fans. The internal fans also exhaust warmed exhaust air into a hot aisle on the other side of the row of racks. In such an arrangement, it is important that the volume of cold air supplied is equal to or greater than that drawn through the servers by their internal fans. If the volume is not sufficient, then the servers can be starved of cooling air or draw in warm air from other areas of the data centre, possibly resulting in the IT equipment overheating. In a traditional data centre in which cooling air is supplied to the cold aisles via an underfloor plenum, it can be difficult to supply the cold aisles with a sufficient volume of cooling air at a given temperature because of the comparatively small cross-sectional area of the underfloor plenum. Furthermore, such underfloor plenums typically have a high air-flow resistance due to their small size, tight corners and the need to force air through perforated floor tiles, thus increasing the amount of energy required to drive air through the system. An approach to overcoming the difficulties of providing large volumes of cooling air through such high-resistance air-flow paths is to provide cooling air at a lower temperature. However, it will be appreciated that providing cooling air at a lower temperature also increases the energy usage of the data centre.
WO2010/139919 and WO2010/139921 (Bripco BVBA) disclose data centres in which cooling air is transported to cold aisles via a corridor at least 1.5 m high, for example a personnel corridor. Entrainment of the cooling air through a corridor having a large cross-sectional area provides a low resistance air flow path enabling large volumes of cooling air to be transported around the data centre at low velocity. Those data centres also make use of ‘free-air’ cooling, in which air from outside the data centre is used as the cooling air. Throughout most of the year, such data centres can be operated with only adiabatic cooling of outside air, thus avoiding the cost of Direct Expansion, or “mechanical” air cooling required with systems that utilise low temperature cooling air.
A disadvantage of such ‘free-air’ cooling methods is that the outside air must be carefully filtered and treated to avoid bringing contaminants such as particulates into the data centre, which contaminants may damage the servers. WO2011/148175 (Bripco BVBA) discloses control processes for data centres utilising ‘free-air’ cooling methods, which processes may comprise a ‘smoke detection’ system which allows the data centre to switch between modes of operation in which outside air is used as the cooling air and modes in which the cooling air consists entirely of air recirculated from within the data centre. It will be appreciated that such a system is relatively complex to operate and remains vulnerable to ingress of contaminated air if detection equipment is deficient or if the time taken for the system to switch mode is too long. Furthermore, when running in ‘full recirculation’ mode, the cooling system typically relies on mechanical cooling to reduce the temperature of hot air returning from the servers.
One approach to making use of ‘free-air cooling’ of a data centre without allowing outside air to come into regular contact with servers is to use a heat exchanger to transfer heat from the warm, recirculated data centre air to cool, outside air. US2010/0300650 (APC) discloses a data centre cooling system comprising an air-to-air heat exchanger in which warm, recirculated air inside the data centre is passed through the inside of a tube as cool outside air is circulated around the outside of the tube. Cooling of the internal recirculated air inside the tube is improved by evaporation of water running over the outside of the tube. A disadvantage of such a system is the relatively high resistance to airflow through the heat exchanger tubes.
US2013/0081784 (AST) discloses a data centre including a passive air-to-air heat exchanger comprising heat tubes. The vertically mounted heat tubes are arranged so that their lower ends are in contact with warm, internal recirculated air from inside the data centre, while their upper ends are in contact with cool, external air drawn through the cooling unit from outside the data centre. The warm, internal air heats the fluid contained in the heat tubes, which evaporates and rises to the top of the tubes, drawing heat out of the internal air. As the cooler, outside air passes over the tops of the heat tubes, the evaporated fluid inside the tubes condenses, releasing heat to the outside air, and the condensed fluid returns to the bottom of the heat tubes. US2015/0034270 (Thermo-Tech) also discloses an air conditioning system for a data centre comprising heat pipes. The air conditioning system includes a sprayer which sprays cooling fluid onto the parts of the heat pipes in contact with the external air to improve the cooling capacity of the system.
A problem associated with known data centre cooling systems using heat exchangers is how to achieve close control of the cooling capacity. It will be appreciated that although it may be desirable to increase the cooling capacity of the heat exchanger by, for example, spraying the parts of the heat exchanger in contact with the external air flow with a fluid (such as water) which evaporates into the external air stream, it is nevertheless costly and inefficient to run such sprayers when they are not needed. Furthermore, introducing substantial quantities of liquid coolant into the system in this way may lead to a degradation of components, for example due to corrosion by the fluid itself or by damage caused by contaminants in the fluid.
The IT industry itself has long recognised the criticality of central computing facilities and the need for energy efficient operations to control cost effectiveness. Current data centre technology is the summation of 30 years of innovation and engineering design thought and has come a long way in recent times. The increasing reliance of organisations of all types on their computing resources has led to data centres often being regarded as ‘mission critical’ facilities, which must be kept online at all costs. To achieve this, data centres are designed to be resilient, often with redundancy built into the design to ensure that if one component or section fails, another can take its place without interruption of the data centre's operation. For example, a data centre's power supply is typically backed up with emergency batteries and/or generators that automatically cut in in the event of a power cut, and uninterrupted power supply (UPS) systems are provided to cover any time interval between loss of external power and start-up of the backup power source. Each aspect of the power system is itself designed to have built-in redundancy, for example by having the power connection to the external power supply duplicated throughout the data centre, and by providing n+1 batteries/generators and UPS devices where n is the number of such devices required for operation of the data centre (or a particular section of the data centre in larger installations) at full load.
It will be appreciated that a data centre's cooling system should also be resilient, since allowing equipment to overheat can rapidly lead to damage to servers and ultimately to failure of the data centre. One approach to providing resilience in data centre cooling systems is to build redundancy into the design by including n+1 air handling units where only n units are required for operation of the data centre (or a particular section of the data centre in larger installations) at full load. Thus, data centres traditionally comprise more air handling units than required for normal operation. It will be appreciated that such an approach is wasteful, both in terms of the data centre footprint (data centre air handling units are typically large, occupying many hundreds of square feet) and in terms of the cost of duplicating all air handling components. For example, an air handling unit, for example an indirect air handling unit, for cooling 200 kW of IT equipment may have a footprint of around 18 m2 and a height of around 4 m, whereas a similar unit with the capacity to cool 390 kW of IT equipment may have a footprint of around 36 m2. Often, this approach is particularly wasteful because only a limited number of the air handler components are prone to failure or require maintenance.
In traditional, direct free-air cooling data centre air handling units, in which the servers are cooled by direct contact with air from outside the data centre drawn through the air handling unit into the data centre, essential components of the air handling unit (such as fans, filter banks, humidification units and back-up mechanical cooling units) can be duplicated within the handling unit so that if, for example, one fan fails or requires maintenance, other fans are capable of maintaining operation of the cooling unit even when the data centre is at full load.
While a similar approach can be taken with certain components of an indirect air handling units, care must be taken to ensure not only that the air handling unit can meet the cooling requirements of the data centre at full load when one or more components fail or are shut down for maintenance, but also that components can be maintained or repaired without mixing the separate internal and external air streams. The latter requirement is particularly challenging for the heat exchange elements (such as heat tubes) of an indirect air handling unit. It will be appreciated that the heat tubes of an indirect air handling unit are in contact with both the internal air stream and the external air stream. Typically, in order to replace one or more heat tubes if they fail or require off-site maintenance, the air handling unit must be shut down.
The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved data centre and an improved method of cooling IT equipment in a data centre.